Since organic EL devices are self-luminescent devices, they are bright and excellent in visibility as compared with liquid-crystalline devices and capable of giving clear display, so that the organic EL devices have been actively studied.
In 1987, C. W. Tang et al. of Eastman Kodak Company put an organic EL device using organic materials into practical use by developing a device having a multilayered structure wherein various roles are assigned to respective materials. They formed a lamination of a fluorescent material capable of transporting electrons and an organic material capable of transporting holes, so that both charges are injected into the layer of the fluorescent material to emit light, thereby achieving a high luminance of 1000 cd/m2 or more at a voltage of 10 V or lower (see e.g., Patent Documents 1 and 2).
Patent Document 1: JP-A-8-48656
Patent Document 2: Japanese Patent No. 3194657
To date, many improvements have been performed for practical utilization of the organic EL devices, and high efficiency and durability have been achieved by an electroluminescent device wherein an anode, a hole-injecting layer, a hole-transporting layer, an emitting layer, an electron-transporting layer, an electron-injecting layer, and a cathode are sequentially provided on a substrate, to further segmentalize various roles (see e.g., Non-Patent Document 1).
Non-Patent Document 1: Japan Society of Applied Physics Ninth Workshop Preprint, pp. 55-61 (2001)
Moreover, for the purpose of further improvement of luminous efficiency, utilization of triplet exciton has been attempted and utilization of a phosphorescent material has been investigated (see e.g., Non-Patent Document 2).
Non-Patent Document 2: Japan Society of Applied Physics Ninth Workshop Preprint, pp. 23-31 (2001)
The emitting layer can be also prepared by doping a charge-transporting compound, generally called a host material, with a fluorescent material or a phosphorescent material. As described in the above-mentioned Workshop Preprints, the choice of the organic materials in organic EL devices remarkably affects various properties such as efficiency and durability of the devices.
In the organic EL devices, the charges injected from the both electrode are recombined in the emitting layer to attain light emission. However, since the mobility of holes is higher than the mobility of electrons, a problem of reduction in efficiency caused by a part of the holes passing through the emitting layer arises. Therefore, it is required to develop an electron-transporting material in which the mobility of electrons is high.
A representative light-emitting material, tris(8-hydroxyquinoline)aluminum (hereinafter referred to as Alq3) is commonly used also as an electron-transporting material. However, since it has a work function of 5.8 eV, it cannot be considered that the material has hole-blocking capability.
As a technique to prevent the passing of a part of holes through the emitting layer and to improve probability of charge recombination in the emitting layer, there is a method of inserting a hole-blocking layer. As hole-blocking materials, there have been hitherto proposed triazole derivatives (see e.g., Patent Document 3), bathocuproine (hereinafter referred to as BCP), a mixed ligand complex of aluminum (BAlq) (see e.g., Non-Patent Document 2), and the like.
On the other hand, as an electron-transporting material excellent in hole-blocking ability, there is proposed 3-(4-biphenylyl)-4-phenyl-5-(4-t-butylphenyl)-1,2,4-triazole (hereinafter referred to as TAZ) (see e.g., Patent Document 3).
Patent Document 3: Japanese Patent No. 2734341
Since TAZ has a work function as large as 6.6 eV and thus exhibits a high hole-blocking ability, it is used as an electron-transporting hole-blocking layer to be laminated onto the cathode side of a fluorescence-emitting layer or phosphorescence-emitting layer prepared by vacuum deposition, coating or the like, and contributes to increase the efficiency of organic EL devices (see e.g., Non-Patent Document 3).
Non-Patent Document 3: Fiftieth Meeting of Japan Society of Applied Physics and Related Societies, 28p-A-6 Lecture Preprint, p. 1413 (2003)
However, TAZ has a great problem of having low electron transport property, and it is necessary to prepare an organic electroluminescent device in combination with an electron-transporting material having a higher electron transport property (see e.g., Non-Patent Document 4).
Non-Patent Document 4: Japan Society of Applied Physics, Journal of Organic Molecules/Bioelectronics Section, Vol. 11, No. 1, pp. 13-19 (2000)
Further, BCP has a work function as large as 6.7 eV and a high hole-blocking ability, but has a low glass transition point (Tg) which is 83° C., so that it is poor in thin-film stability and thus it cannot be considered that it sufficiently functions as a hole-blocking layer.
All the materials are insufficient in thin-film stability or are insufficient in the function of blocking holes. In order to improve characteristic properties of the organic electroluminescent devices, it is desired to develop an organic compound which is excellent in electron-injection/transport performances and hole-blocking ability and is highly stable in a thin-film state.